Derivative control of servo systems



Oct. 21, 1952 H. TOOMIM DERIVATIVE cormox. OF SERVO SYSTEMS 3 Sheets-Sheet 1 Original Filed March 6, 1945 .2 f'fh me or sfaf'or 8 M Armafure or keeper or jm.

F/lgjc A-C. JOUrce INVENTOR. Hers/)e/ 730011721 ATTORNEYS H. TOOMIM Oct. 21, 1952 DERIVATIVE CONTROL OF SERVO SYSTEMS 3 Sheets-Sheet 2 Original Filed March 6, 1945 3 RECEIVER TRANS ITTER 2" Source J AG. source.

INVENTOR. Hersfie/ 750mm BY and? ATTORNEYS Oct. 21, 1952 H. TOOMIM 2,615,149

DERIVATIVE CONTROL OF SERVO SYSTEMS Original Filed March 6, 1945 w 3 Sheets-Sheet 3 REGf/l/ER Fig. 5-

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TRANS lf/ 77TH THAN/SN/LTER RECEIVER a I n v i 2'0 T Q I I0 20 23 \msmfi l /6 24 ya; la

INVENTOR. V g I Hersfle/ 7517/77/07 2 wew ATTORNEYS of mechanical elements of the system.

Patented Oct. 21 1952 UNITED v STATES PATENT 1 OFFICE 2,615,149

DERIVATIVE. CONTROL OF SERVO SYSTEMS I l I Hershel Tocmim, Douglaston, N. Y. 1 Original application March 6, 1945, Serial N...

581,296. Divided and thisapplication February 10, 1950, Serial No. 144,561

. 1 The present application is a division of my application Serial No. 581,296, filed March 6, 1945, and entitled Derivative Control of Servo Systems. I "In a signal transmitting system it is often desirable to transmit and utilize not only the primary signal itself, but derived information concerning its rate of change, direction of change, etc. 7 For example; a system for transmitting informationpertaining to the angular-displacement of a shaft from a given setting may involve a transmitter synchro connected to the said shaft, 8. receiver synchro, an amplifier, and a'mechanical means of driving the shaft of the receiver synchro, together with any'apparatus connected thereto, to the position called for by the transmitted signal. Due to mechanical inertia, such a system would be liable to hunting or oscillation at its natural mechanical resonant frequency. Circuits are therefore added to the amplifier of such a system for obtaining a derivative signal whose value and phase depend upon the rate and direction of change of the displacement or error signal. The function of such a derivative signal is twofold: I

' 1. To provide additional torque when the error signal is increasing in order to overcome inertia 2. To provide negative or reverse torque as the mechanical system approaches the called for position of the receiver rotor and apparatus geared toits shaft, in order to overcom inertia of the moving parts, which would make the system overshoot and hunt. e In the simple system described, only the first derivative has been considered- In practice, two

or more derivatives of the error signal may bev required to be added to the error signal for correct operation of the mechanical system it is desiredtoposition.

The use of derivatives ina servo; system is described, for example, in copending application of Francis H. Shepard, Jr., Serial :No. 499,774, filed August 24, 1943, for Follow-Up Device. A- study of present systems forobtaining derivative signals reveals that considerable complication of the amplifier circuits is necessary and that the derivatives obtained by such circuits are good'only up to some relatively'limited rate of change of the error signal, beyond which they lose their desired 90 degree phasing. This invention involves the introduction of direct current fields into selsyn units to obtain, derivatives directly.

6 Claims. (Cl. 318-24) -Methods for producing voltages in conductors by means of moving magnetic fields are characterized by simplicity and linear operation which faithfully represents all velocities of the relative motions inducing the voltages. This application covers several variations of related systerns intended to combine the advantages of movement-generated D. C. derivative signals with position-generated A. C. error signals in servo systems.

Accordingly, it is an object of this invention to provide a simple means for producing one or more D. C. derivatives of a signal in a servo mechanism.

. A feature of my invention is that the derivatives areproduced independently of frequency. In other words, the derivatives will beaccurate 1y produced'despite the rate of change of the error signal. i ,Another feature of my invention is that my simplifiedm'eans for obtaining a first D. C. derivative permits second and other subsequent derivatives to be derived easily and with more simple circuits than have heretofore been neces y- In the drawings, Figure 1 isa simplified'circuit diagram of one form which my invention might take, and which is primarily useful in permitting a simple explanation of the principles involved;

Figures 1a to 1d are curvesillustrating the operation of Figure 1; and I v Figures 2 to .7 ar e circuit diagrams of other forms of my invention, Figures 4' and 5 being the forms which are preferred for practical use at the present time. I

In the drawings similar reference numerals refer tosimilar or corresponding parts in the various figures. 1

Referring to Figure 1, -I have shown an A. C. source I connected in series with a D. C. source 2 through a coil 3 which may be rotatable with respect to a'second coil '4 in inductive relation therewith. I have also shown a. second sourceof D. C..voltage 5, connected through a resistance 4 I, which may be used alternatively to thesource 2 as by means of a switch 5|. I I

In simplest form, my invention would consist, as shown in Figure 1, in applying a D. C. component'to the rotor orstator of a single-coil rotor, single-coil stator pickup or rotatable transformer.

It is, illustrated as appliedito the rotor 3. Move- I D. C. component depending on the rate and direction of motion (d sin dt varying the axis of the A. C. signal above or below zero or ground potential. This is shown graphically in the idealized oscillograph traces in Figures la to 1d, representing the case where the rotor or stator has been displaced to one side of the null.

rotor 3 through a certain intervalof time,.thedisplacement being indicated on the. vertical axis and time on the horizontal axis, and the partic Figure 2 shows my invention applied to a standard E transformer or electromagnetic signal pickoif unit consisting of an E-shaped iron core 8 carrying three windings 9, I0 and II. The central winding In or primary provides the A. C. and D. C. fields. The two outside windings, 9 and II, yield an A. C. voltage of phase and amplitude proportional to the direction and amount. of displacement of the armature or'keeper I2. D. C. voltage or current is also induced in the output windings, its amplitude and polarity depending upon direction andrate of motion of the armature and the direction of the D. C. field current.

This D? C. voltage is therefore the first derivative of the error or displacement signal.

It'will be understood in the circuit of Figure 2 thatthe. armature l2 may be displaced in one ular displacement illustrated being less than 90 I degrees.

Figure lb illustrates the voltage which would be inducedin the secondary '4 during this interval of time if no'D; C. were present. It will beobserved thatfor a small angular displacement of the rotor 3 there is a small A. C. voltage induced in the secondary 4 which increases in direct'pro portion to the angle of: displacement. As the rotor 3'is' returned to its" original position the A. C. voltage induced in the secondary accordingly falls" until it returns tozero.

Figure 1c. illustratesthe voltage which is inducedinto the secondary 4 because of the movement of 'the rotor 3. just referred to due' tothe presenceofthe D. C. voltagesource'2 or 5 in the circuit of coil 3. Due to this D. C. there-will be steady current flowing in the coil 3' causinga steadyfiux inthis coil'which when coil 3* is stationary willinduce no voltage" in the'secondary- 4' because it is not changing. When'the position of coil 3 changes there will be a changein the lines offorce. which cut the coil 4 and there will be induced therein a voltage illustrated in Figure 1c bythe curve 6; This'curve; as1illustrated, rises to a positive value and descendsto: zero during the time that the coil 3 is moving in one direction:

The curve represents velocity of the coil 3 andis" therefore the first derivative of the position of this. coil and; as explained above, provides a secondary voltage which is" useful in the operation of a servo system; Similarly, whenthecoil 3 is replaced to its. zero position the D. C. magnetic field will induce in thesecondary 4 a voltage illustrated by the curve 'I-inFigure-1c-which is equal and opposite to the curve 6 and will be useful in'a servo system forreasons whichhave already been described.

Figure 1d illustrates the total voltage induced in the secondary '4. Thatis; it'is the-combination of the voltages of- Figure 1b with those of Figure 1c.

The-circuit'shown-in Figure=1 is a rotatable transformer used alone, the voltage output of whichcould be fed-tosuitable amplifying equipment which, for example, might" be used to drive a motor to repositionthe rotor: shaft back to its null position, or drivethe rotor shaft to a new position called for by a -new positionzof: the stator. In this latter case, the system-would bea: complete torque amplifying-system used; for'example, to drive and position cumbersome mechanical equipment in response to a-verylight and delicate force-acting through a smalldistance. A typical application would be the positioning of the control surfaces of anaircraft in response to relative motion of theaircraft and small gyroscopes used as attitude references;

direction or the other by the device which creates the error signal, such as a gyro device in the case of an automatic pilot, while the E frame 9 can be fixed to the body of aplane, thus creatingrelativemovement between the frame and its armature depending on a change in direction of the plane. The error signal thus created could then be used as-explained above to provide a restoring force to maintain the plane. in a standard desired. condition offlight.

The'circuit of Figure 3 showsthe D. C. source applied to the transmitter of a conventional. selsyn system such as shown in the Shepard application referred to above. In such a system the error signal induced in the coil 13 may be amplified and caused to operate a suitable mech-- anism which it is desired to control, and also to rotate the coil 13 to a position correspondingto, that of the coil 3. That is, desired motion is indicated bythe movement of. the coil 3 by the operator of the device, and this causes. the desiredmotion. while also moving the coil I3 to'a new position to reposition the coil l3 to zero output when the desired motion. has been; completed.

Since the constant D. C. field of the transmittenrotorcuts the windings'of the stator during relation motion of the windings, a firstderivative of the displacement signal will; beinduced in the stator windings; With selsyn-units, a second derivative of: the displacement. signal willbe found mixed with the displacement signal. in the output ofthereceiver rotor.

Figure.- 4. there is illustrated a. preferred form of my invention: in; which there-is noseparatet D; C.. source, but: in: which the. D. C.'. is

produced within. the. selsyn. system. by. the. use. of rectifiersz. Such. rectifiers: are illustrated at 20, 2| and 22. The transmitter:- selsyn is'iilustrated bythe rotor winding;3' and statorwindings 14",. I5. and I6 whileithez receiver selsynzis illustrated. by the rotor: winding l3 and: cone. spending. stator; windings I1, l8' and [9. The circuit: also includes. three condensers illustrated at 23, 24 and 25.

Referring now to. Figure 4, in order; for. the motion-generated. D. C. to. be in:accordance. with the equation Ebr-D. C.=d. sin: (01-02) the posi-: tion and sense of; the..D;. C; fl'eldmust correspond with the A.. C. field; regardless of. the new: null positions to which. the rotor of. the selysnrtransmitter may: be. moved. This. is' accomplished automatically in the. case. of the applicationof the D. C'. to:therotor: as; in; Figure 3. However, as stated above, such aprocedure inthe caseior a. complete. transmitter: and? receiver selsyn. syss tem, provides second; rather thanfirst' derivative voltage: of. position (not: error or difference).- in. the output of the receiver.

Thecircuit of Figure 4 shows a method for providing a D. C. field which yields a first derivative voltage, both 'of transmitter or displacement signal and. of rebalancing or receiver re-positioning signal. The difference of these two signals constitutes the error signal E (01-02) and the resulting derivative will be of 01-.02. The D.- C. field at all times assumes the position and sense of the A. C. field. This'circuit requires thatthe midpoint orjunction of the stator windings be brought out to a separate terminal. A. C. input is applied to the rotor and to the neutrals 0 and 0'. It should be noted that, current may fiow only in one direction inthe stator. windings of the circuit ofFigure4. However, the A. C. error or displacement signal is transmitted quite effectively in terms of therelative. ratios of the amplitudes of the half-cycle pulses transmitted andreceived. This rectification has the eflect of introducing large second, and other harmonics into the system but does not inter fere with the accurate operation of the system in transmitting the displacement signal. Since the relative amplitudes of the undirectional current pulses through the stator windings of both transmitter and receiver are governed by the position of the rotor of the transmitter, theangular, position of the.D. C. field in both units is'likewise dependent upon the position of the transmitter rotor.v The capacitors 23, 24 and provide ameans for controlling the ratio of D-.-C. to A. C. in the selsyn receiver windings, and may have any value from zero to 2 to 3 microfarads or even-more. g

It is necessary to apply A. C. input between the neutrals 0 and 0' in order to phase the diode rectifiers properly to provide rectification of current through any particular diode at one position of the rotor 3 and avoid rectification 180 degrees therefrom. Without this A. C. input, the D. C. field would rotate twicefor one rotation of the transmitter rotor. This may be understood by considering the phase relationships in any pair of stator windings forinstance the windings l6 and I8. Ata particularposition of the rotor 3 which may be considered a position of zero degrees with respect to winding l6 there will be an alternating current induced in the winding H; which will be in'phase with the voltage supplied to this winding directly from the A; C. source I. The forward half cycle of this current will pass through the rectifien-ZL-through the winding l8 and back to the source I. For 180 rotation of the coil 3, however, the current introduced in the coil I6 will be directly out of phase with the current supplied directly from the source I. Accordingly, on the forward half cycle of induced current the A. C. current supplied directly from the source I will be opposite to that induced from coil 3 to coil l6, so there will be less current through the rectifier for this position of the coil 3. The same explanation applies to each of the other windings for corresponding positions of the rotor 3 with respect to them. Accordingly, if the rotor 3 were continuously rotated there would be pulses of forward current through each rectifier 20, 2| and 22 which would reach a maximum once for each complete rotation of the coil 3. Therefore, the direct current field in the stator windings in the transmitter and receiver selsyn will always correspond in angular posiiton with the A. C. field induced by the rotor 3. Accordingly, there will, as before, be a signal induced in the receiver rotor I3 by and proportional to the first'derivative of the relative motion of rotor 3 to the transmitter stator windings as .well as by relative motion of the rotor. 13 to the receiver stator windings. v

If no neutrals are available or if it is desired to isolate the transmitter and receiver or receivers for D. C., the circuit of Figure 5 may be used. In this figure there are shown resistors 26, 21 and 2.8 and transformers 29, 3D and 3| having respectively, primaries 32, 34. and 36 and secondaries 33, 35, and 37. In this circuit threeY-connected resistors 26, 21 and 28 serve as a false neutral, and three transformers29, 30 and 3! isolate the circuits-for D. C. The three diode rectifiers 20, 2! and 22 provide the D. C. component,,-.while capacitors 23, 24 and 25 control the ratio of D. C. to A. C.-,field strength. The D. o. field set up by the stator windings of the receiver follows in space and is superimposed on the A. C. field resulting from the positionof the transmitter rotor. I

The circuit of Figure 6 illustrates the use of two D. C. components for the production inthe output of a combined error signal, its first derivative, and the second derivative of the transmitter signal. The circuit isexactly similar to that of Figure 4, except for the addition of the battery or other D. C. source in series or parallelwith the A. C. source used to excite the rotor of ,the transmitter, as in Figure 3.

I Thus Figure 6 is a combination of Figures 3 and 4. In the case of Figure 3 there is produced in the coil 13 the second derivative of motion of the coil 3 and this is also produced in the coil I3 of Figure 6. In the case of Figure 4there is produced. in the coil 13 the first derivative of motion oftfie coil 3 as well as the derivative of mo: tion of coil l3, and this also appears in coil [3 of Figure 6. Therefore,in Figure 6, motion of the transmitter rotor 3 with respect to its stator windings will produce in output coil 13 first and second derivatives of said motion and motion of coil l3v with respect to the receiver stator windings will produce in coil 13 first derivative of said motion. In both cases A. C. error signal will appear in the coil I3.

Direct current could similarly be applied across the rotor 3 of. Figure 5, with the same results except that wherea second derivative may be obtained in Figure 6, a third derivative may be obtained in Figure 5 with such a D. C.,s ource added- Y L In the manufacture of signal pickoff units, transmitters, or receivers, some of the desired effects could be obtained by introducing a fixed amount of permanent, polarized magnetization into the rotors. In some applications this would be equivalent to introducing a direct current from an external source.

One further advantage of this method of obtaining a D. C. derivative directly from the synchro transformer output is that second and other subsequent derivatives may be derived easily and with more simple circuits than are required in securing derivatives of A. C. derivatives.

In Figure 7 I have illustrated a simple circuit for use in a servo system in which A. C. and D. C. are combined through a potentiometer having a tap which provides the error signal. This system has the advantage of permitting the use of an A. C. error signal which has many advantages but it also permits the use of a simple derivative circuit such as condenser 42 and resistance 43 by means of which the first derivative of position may be obtained due to the presence of the D. C.

adieu-4e 7 current from the source .2. Thus, in this circuit I introduce a direct current into the device from which an A. C. error signal is derived for the purpose of simplifying the taking .01 derivative voltages.

Figs. 2, 3, and 6, a D. C. source and resistor 4| may be shunt connected as in Fig. 1, in place of source 2.

It will be understood that the various values and arrangements illustrated are illustrative and that my invention is not restricted to the particular, specific details shown. It will also be understood by those skilled in the art that my invention is capable of various modifications. I do not desire therefore to be restricted to the particular details shown and described but only within the scope of the appended claims- What is claimed is:

1. A servo system comprising a transmitter selsyn, a receiver selsyn, a source of alternating current connected across the transmitter rotor and across the neutral points of the transmitter and receiver selsyns, and a rectifier between each winding of the transmitter selsyn and its corresponding winding on the receiver selsyn.

2. A servo system comprising a transmitter selsyn, a receiver selsyn, a source of alternating current connected across the transmitter rotor and across the neutral points of the transmitter and receiver selsyns, a rectifier between each winding of the transmitter selsyn and its corresponding winding on the receiver selsyn, and a condenser connected across each pair of windings of the receiver selsyn,

3. A servo system comprising a transmitter selsyn, a receiver selsyn, a source of alternating current connected across the rotor of said transmitter, and also connected across the neutral points of said transmitter and receiver selsyn stators, a source of direct current connected to said transmitter rotor, and a rectifier connecting the outer end of each transmitter stator winding with the outer end of each corresponding receiver stator winding.

4. A servo system comprising a transmitter selsyn, a receiver selsyn, a source of alternating current connected across the rotor-of said transmitter, and also connected across the neutral points of said transmitter and receiver selsyn stators, a source of direct current connected to,

said transmitter rotor, a rectifier connecting the outer end of each transmitter stator winding with the corresponding outer end of each receiver stator winding, :and a condenser connected across each pair of receiver stator windings.

5. A motion transmitting system comprising a transmitter synchro, .a receiver synchro coupled to said transmittersynchro, reach oi-said-synchros comprising :a polyphase winding having a neutral junction, a plurality. of rectifier-s, each winding of said receiver polyphase winding being connected to a corresponding winding 01 said transmitter polyphase winding in series with a rectifier, and means in impressing an alternating voltage between said neutral Junctions whereby a direct currentfield is produced in said'windings.

6. A transmitter for .a motion trarmnitting system adapted to be connected to a receiver and comprising a rotor 'elementadanted to beiadjmted in position in correspondence with a driving member, a stator element cooperating 'withsaid rotm' element, one of said elements having a .lingie phase exciting winding, the other oi saidelemenis having a polyphase output winding inductively coupled tosaid single phase winding and adapted to be connected to said receiver, meanssupplying alternating current to said single phase winding to produce an alternating magnetic field, and means super-posing a direct current magneticfield on said alternating magnetic field, said last means comprising a rectifier connected in series with each winding of said polyphase winding and a source of alternating current-connected to laid polyphase winding to be rectified by said rectifiers, whereby said polyphase winding produces alternating signals representing both the :displacement of said driving member from a -datum position and a time derivative of saiddisplacement. 1

'HERSHEL TOOMIM.

REFERENCES CITED France. June 1,1934 

